Is there a way to erase all image data blocks for a specific .blend?
When you have a data block that has no users it will automatically be removed when you save your project and reload the file.

Blender does not delete data right away as someone may have a use for it still. An example is if something has been imported/linked/appended and has not been assigned to an object yet. Obviously you want it to stay in memory until it is given a user or else it would be gone as quickly as it was loaded!

To recap simply save your scene and reopen it and all your unused content will go away. With the exception of data that you have specifically told blender not to delete even if it has no users. (You can do that by pressing the button with the letter 'F' on it.)